10476 J. Am. Chem. Soc., Vol. 118, No. 43, 1996
Bennett et al.
(2-Bromophenyl)diphenylphosphine, o-BrC6H4PPh2, was made either
from the PdCl2(NCMe)2-catalyzed reaction between o-bromoiodoben-
zene and diphenyl(trimethylsilyl)phosphine, Ph2PSiMe3 (yield 61%)63
or, less conveniently, by treatment of o-BrC6H4PCl2 with PhMgBr (2
equiv).60 Mp 110-112 °C. 31P{1H} NMR (CD2Cl2) δ -5.2 (s).
(2-Bromophenyl)diethylphosphine, o-BrC6H4PEt2. To an ice-
cooled solution of EtMgI (0.06 mol) in ether (50 mL) was added slowly
o-BrC6H4PCl2 (8 g, 0.03 mol) in ether (60 mL), the temperature being
kept between 0 and 5 °C. After being stirred overnight at room
temperature, the mixture was treated with a deoxygenated, saturated
solution of NH4Cl (95 mL). After addition of some water to give two
layers, the ether layer was dried (Na2SO4), solvent was evaporated,
and the residue was distilled in vacuo. The product was collected at
111-112 °C/0.8 mm (lit.64 90-91 °C/0.15 mm). The yield was 5.4 g
(70%). 31P{1H} NMR (CD2Cl2) δ 15.4 (s).
Bis(µ-(2-diphenylphosphino)phenyl)digold(I), Au2(µ-C6H4PPh2)2
(1a). A solution of o-BrC6H4PPh2 (2.0 g, 5.9 mmol) in ether (40 mL)
was treated with a 1.6 M solution of n-BuLi (3 mL) over 30 min to
give a colorless precipitate of o-LiC6H4PPh2‚Et2O,65,66 which was
washed with hexane. The yield was 1.4 g (5.1 mmol, 87%). This
solid, suspended in ether, was added via cannula to a suspension of
AuBr(PEt3) (1.6 g, 4.1 mmol) in ether (25 mL), the temperature being
kept below -65 °C. The temperature was then held at -40 °C for 2
h and at room temperature for 1.5 h. The off-white solid was separated
by filtration and washed successively with ether (3 × 20 mL), methanol
(15 mL), and hexane (3 × 20 mL). It was then extracted with hot
dichloromethane (300 mL) and the extract was filtered through Celite.
Evaporation gave 1a as a colorless, air-stable solid (1.2 g, 63%), mp
216 °C dec.
Bis(µ-(2-diethylphosphino)phenyl)digold(I), Au2(µ-C6H4PEt2)2
(1b). The solution obtained by addition of n-BuLi to o-BrC6H4PEt2
was added to a suspension of AuBr(PEt3) in ether. Workup as described
above gave 1b in ca. 60% yield, mp 228 °C; it is more soluble than 1a
in CH2Cl2, THF, and toluene. 1H NMR (CD2Cl2) δ 1.5 (dt, 3JPH ) 19,
JHH ) 7, CH3), 2.1 (m, CH2), 7.1-7.8 (m, C6H4).
Dihalodigold(II) Complexes, Au2X2(µ-C6H4PR2)2 [R ) Ph, X )
I (2a), Br (3a), Cl (4a); R ) Et, X ) I (2b), Br (3b), Cl (4b)]. A
stirred suspension of 1a or 1b (0.138 mmol) in dichloromethane (20
mL) at -70 °C was treated dropwise with a solution of iodine, bromine,
or PhICl2 (0.142 mmol) in dichloromethane, the temperature being kept
below -65 °C. In the case of iodine, toluene could also be used as
the reaction medium. After addition was complete, the flask was
shielded from light; the solution was then stirred at -70 °C for 1 h, at
-40 °C for 30 min, and finally at room temperature for 1 h. The
volume of the solution was reduced to about half under reduced pressure
and hexane was added. Precipitation of the product was completed
by cooling the solution to 0 °C. The solid was filtered, washed with
hexane, and dried in vacuo. Yields were 60-80%. The chloro
complexes 4a and 4b were also made by addition of chlorine in CCl4
to occur from a three-coordinate intermediate AuR3 formed by
dissociation of PPh3.57,58 The groups that are eliminated occupy
cis positions and the elimination is a symmetry-allowed process
from a Y-shaped AuR3 molecule.58 Since the AuIII-C bonds
in 19 or 20 are mutually trans, distortion is required before they
can reach a geometry suitable for reductive elimination to occur;
this process may represent one of the activation barriers to be
surmounted. The formation in the rate-determining step of salt-
like species such as 19 or [AuIII(o-C6H4PPh2)2][AuIX2], appears
to be ruled out by the fairly small dependence of isomerization
rate on solvent polarity, although the cation in the latter case is
isoelectronic with the stable platinum(II) complex Pt(o-C6H4-
PPh2)2.49
Migration of the aryl group between the gold atoms in 2-4
may proceed via a two electron-three center intermediate or
transition state such as 22 (Scheme 6). There are many
structurally characterized examples of gold(I) compounds
containing bridging aryl groups,4,6 but none are known for gold-
(II) or gold(III). It is noteworthy that the isomerization is
initiated by reversible loss of the more polarizable anions I-
and Br-, less easily by Cl-, and not at all by the excellent
leaving groups OCOCH3-, OCOPh-, or ONO2-. One possible
explanation is that the heterovalent formulations 18-20 are
disfavored by the presence of hard, strongly electron-withdraw-
ing axial ligands. Conversely, they should be favored by good
electron donors; in support of this idea, we have found that the
dimethyl complexes Au2Me2(C6H4PR2)2 (R ) Ph, Et) adopt
structure 18 (X ) Me) containing gold(I) and gold(III).59 These
compounds, however, do not isomerize to Au2Me2(µ-R2-
PC6H4C6H4PR2), an observation that indicates the importance
of an ionizable leaving group in promoting the C-C coupling.
The longer Au-Au bond in Au2I2(µ-C6H4PPh2)2 (2a) relative
to that in Au2(O2CPh)2(µ-C6H4PEt2)2 (5b) may also be a
significant factor, since the gold atoms have to be separated
further in the coupled product.
Experimental Section
General Procedures. Most syntheses were performed under dry
nitrogen with use of standard Schlenk techniques, although the solid
complexes were air-stable. Reactions with gold complexes were carried
out in vessels shielded from light. The following instruments were
used for spectroscopic measurements: Varian XL-200E (1H at 200
MHz, 31P at 80.96 MHz), Varian Gemini-300 BB (1H at 300 MHz, 31
P
at 121.4 MHz), Varian VXR-300 (variable temperature, 1H at 300 MHz,
31P at 121.4 MHz), VGA Micromass 7070F (medium resolution EI
mass spectra), VG ZAB-2SEQ (high resolution EI and FAB mass
spectra), Perkin Elmer PE 683 and 1800 (infrared spectra as KBr disks
to 1a or 1b. 2b-8b: 1H NMR (CD2Cl2) δ 1.3 (dt, JHH ) 7.5, JPH
+
JP′H ) 19), 2.6 (m, CH2), 6.9-8.3 (m, C6H4); 2b {31P}(C6D6) δ 0.95
(t, JHH ) 7.5, CH3), 1.75 (sxt, sepn ) 7.5 Hz, CHH), 2.11 (sxt, sepn
) 7.5 Hz, CHH).
or Nujol mulls in the ranges 4000-200 and 4000-150 cm-1
,
respectively), and Hewlett Packard HP 8450 (UV and visible spectra).
The NMR chemical shifts (δ) are given in ppm relative to TMS (1H)
and to 85% H3PO4 (31P), both referenced to solvent. Coupling constants
(J) are given in hertz. Elemental analyses and molecular weight
determinations by osmometry in dichloromethane at 25 °C (Knauer)
were performed in-house. These data together with the 31P NMR
chemical shifts are given in Tables 2 and 4.
Bis(benzoato)digold(II) Complexes, Au2(O2CPh)2(µ-C6H4PR2)2 [R
) Ph (5a), Et (5b)]. A stirred suspension of 1a or 1b (0.163 mmol)
in CH2Cl2 (40 mL) was treated with an excess of solid benzoyl peroxide
(0.245 mmol). The mixture, shielded from light, was set aside at room
temperature for 48 h, and solvent was removed under reduced pressure.
The yellow residue was stirred with ether for 2 h to remove the excess
of benzoyl peroxide and again pumped to dryness. The solid was
dissolved in the minimum quantity of CH2Cl2, hexane was added, and
the mixture was set aside at 0 °C. The solid products were separated
by filtration and washed with hexane. Yields were 80-90%. Crystals
of 5b suitable for X-ray structural analysis were obtained from CH2-
Cl2/hexane.
Other Au2Y2(µ-C6H4PR2)2 Complexes [R ) Ph, Et; Y ) OAc
(6a, 6b), ONO2 (7a, 7b)]. The dihalodigold(II) complexes 2a-4a or
(63) Tunney, S. E.; Stille, J. K. J. Org. Chem. 1987, 52, 748.
(64) Hart, F. A. J. Chem. Soc. 1960, 3324.
(65) Hartley, J. G.; Venanzi, L. M.; Goodall, D. C. J. Chem. Soc. 1963,
3930.
Starting Materials. The compounds o-BrC6H4PCl2,60 PhICl2,61 and
AuBr(PEt3)62 were prepared by the appropriate literature procedures.
(57) Tamaki, A.; Magennis, S. A.; Kochi, J. K. J. Am. Chem. Soc. 1974,
96, 6140.
(58) Komiya, S.; Albright, T. A.; Hoffmann, R.; Kochi, J. K. J. Am.
Chem. Soc. 1976, 98, 7255.
(59) Bennett, M. A.; Welling, L. L.; Hockless, D. C. R. unpublished
work; similar behavior is observed in the bis(ylide) series, see refs 7 and
12.
(60) Talay, R.; Rehder, D. Z. Naturforsch. B 1981, 36, 459.
(61) Lucas, H. J.; Kennedy, E. R. Organic Syntheses, Wiley: New York,
1955; Collect. Vol. III, p 482.
(62) Coates, G. E.; Kowala, C.; Swan, J. M. Aust. J. Chem. 1966, 19,
539.
(66) Harder, S.; Brandsma, L.; Kanters, J. A.; Duisenberg, A.; van Lenthe,
J. J. Organomet. Chem. 1991, 420, 143.
Bennett, Martin A.
